Emissions regulations for internal combustion engines have become more stringent over recent years. Environmental concerns have motivated the implementation of stricter emission requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Generally, emission requirements vary according to engine type. Emission tests for compression-ignition (diesel) engines typically monitor the release of diesel particulate matter (PM), nitrogen oxides (NOx), and unburned hydrocarbons (UHC).
Exhaust aftertreatment systems receive and treat exhaust gas generated by an internal combustion engine. Typical exhaust aftertreatment systems include any of various components configured to reduce the level of regulated exhaust emissions present in the exhaust gas. For example, some exhaust aftertreatment systems for diesel powered internal combustion engines include various components, such as a diesel oxidation catalyst (DOC), particulate matter filter or diesel particulate filter (DPF), and a selective catalytic reduction (SCR) catalyst. In some exhaust aftertreatment systems, exhaust gas first passes through the diesel oxidation catalyst, then passes through the diesel particulate filter, and subsequently passes through the SCR catalyst.
The SCR catalyst in an exhaust aftertreatment system reduces the amount of nitrogen oxides (NOx) present in the exhaust gas. Generally, the SCR catalyst is configured to reduce NOx into constituents, such as N2 and H2O, in the presence of ammonia (NH3). Because ammonia is not a natural byproduct of the combustion process, it must be artificially introduced into the exhaust gas prior to the exhaust gas entering the SCR catalyst. Typically, ammonia is not directly injected into the exhaust gas due to safety considerations associated with the storage of gaseous ammonia. Accordingly, most conventional systems are designed to inject a liquid reductant (e.g., diesel exhaust fluid (DEF), ammonia, metal chloride salt, etc.) into the exhaust gas, which is capable of decomposing into gaseous ammonia in the presence of exhaust gas under certain conditions. The liquid reductant commonly used by conventional exhaust aftertreatment systems is DEF, which is a urea-water solution. Such SCR systems typically include a liquid reductant source and a reductant injector or doser coupled to the source and positioned upstream of the SCR catalyst. The reductant injector injects liquid reductant into a decomposition space or tube through which an exhaust gas stream flows.
Recently, due to the safety and environmental considerations associated with the storage of gaseous ammonia, some SCR system manufacturers utilize solid ammonia storage to provide the ammonia necessary for NOx reduction on the SCR catalyst. Generally, such systems include a storage cartridge storing solid ammonia and a heater for transforming the solid ammonia into a gaseous state preparatory for injection into the exhaust gas. Because solid ammonia stored in the storage cartridge is consumed once transformed into a gaseous state and injected into the exhaust gas, the solid ammonia must be replaced or replenished in order to continue the NOx reduction. Some systems require a physical replacement of an empty storage cartridge with a full storage cartridge. Rather than completely replace an empty storage cartridge, some systems, such as those employing a two stage storage system including solid ammonia in at least one cartridge of the two stage system, replenish or refill an empty or partially empty cartridge via an ammonia storage regeneration event. In such an ammonia storage regeneration event, the second stage cartridge that still contains solid ammonia may be heated to release gaseous ammonia into the empty or partially empty storage cartridge. Although both one and two or multi stage solid ammonia systems may adequately provide ammonia for NOx reduction, such systems commonly employ inefficient timing strategies or triggering events for gaseous ammonia generation (in a single stage system) and ammonia storage regeneration (in a two or multistage system). For example, some systems regenerate an ammonia storage cartridge based solely on an estimated ammonia storage level of the cartridge.